In all, there have been at least twelve large explosive volcanic eruptions, but the configuration of the present, roughly semicircular island is the result of an enormous volcanic explosion which occurred probably around 1630 BC, literally blowing the top off the island and changing what had been a typical half-submerged mountain of an Aegean island into a flooded crescent caldera, in the middle of which a few small smoking islands still bear witness to volcanic activity.

Some have speculated that this event was the inspiration for the myth of Atlantis. The towns of Fira, Ia (also known as Oia) and Thirasis cling to the steep cliffs facing into the caldera bay.

Scientists believe that about two million years ago, volcanoes located at the bottom of the Aegean Sea west of Santorini began to generate volcanic emissions, which resulted in the formation of numerous small islands, and eventually formed two colossal shield volcanoes, which are conical mountains. These conical mountains have teamed up with non-volcanic islands to make way for one large island, Santorini. The volcanic arc of Santorini is approximately 500 km (300 mi) long and it extends all the way from the Greek mainland to the Turkish Bodrum peninsula.

But as explorations show, Santorini did not begin as a volcano. Its basement rocks consist of Mesozoic and Early Cenozoic-aged marble and phyllite. Only later volcano explosions corrected today’s forms of the island, so, therefore, is considered a volcanic island. The most ancient rocks openings are at the ancient city of Thira, located in the southeast portion of the island. All Thera is built upon a small, non-volcanic basement. The younger volcanic rocks can be found on the islands in the middle of the caldera. The current caldera was formed about 3600 years before, during the most famous Minoan eruption.